Multistage countercurrent recrystallization process and apparatus for performing same

ABSTRACT

A multistage process for the separation and purification of a desired crystalline material by repeated dissolution and recrystallization, wherein crystals and solvent move countercurrent to one another through the stages. A fluid temperature gradient is maintained within each recrystallization unit to make use of convection to facilitate or accelerate concurrent dissolution and recrystallization within a single vessel. Solid recrystallized material is moved from one unit to the next unit. An automated apparatus and a manually operated apparatus for carrying out the process are also described.

BACKGROUND OF THE INVENTION

The present invention relates to a method for separating and purifying adesired crystalline substance from a solid mixture and for removingoccluded impurities by repeated dissolution and recrystallization. Italso relates to apparatus suitable for use in practicing the method.

Because of the structure of the lattice cell, crystallizationtheoretically offers the advantage over other separation processes ofyielding a pure component in a single stage. In practice, however, it isgenerally not possible to obtain a pure component by a singlecrystallization because of occlusions of mother liquor. Theseocclusions, or small pockets of solution trapped within or betweenrelatively large individual crystals, can be removed by redissolving theimpure crystals in pure solvent. The pure solvent dilutes impuritiesfrom the occluded solution and, upon subsequent recrystallization, ahigher degree of purity is achieved. Consequently, multiplecrystallizations can provide any desired degree of purity, limited onlyby the purity of the solvent.

Most production crystallizers for commodity chemicals are large,expensive, automated devices which achieve supersaturation in continuousoperation, either by cooling of hot concentrated solutions or byevaporation of solvent within the crystallizer. Prolonged suspension ofgrowing crystals in agitated mother liquor effectively minimizes thenumber and extent of occlusions and provides some measure of particlesize control. Attempts to stockpile crystal and mother liquorinventories of intermediate purity to achieve multistage countercurrentcrystallization with a single unit become increasingly unattractive asthe material value and the number of required crystallizations increasebecause of scheduling complexity, storage requirements, and carryingcosts.

Both the evaporative and the cooling type commercial crystallizersrequire a substantial investment in energy in order to recover anycrystals, and this cost penalty is multiplied by whatever number ofrecrystallizations are required for achieving the desired level ofpurity. Furthermore, addition of more water to redissolve purifiedcrystals for succeeding crystallization steps multiplies the waterpurification cost and/or the contribution of soluble impurities in thiswater to the ultrapure crystal.

Daily feed batch and mother liquor sampling and very prompt analyses ofthese samples for impurities are generally required for control ofproduct purity levels during operation of a continuous crystallizer. Theresults of the various analyses are considered during calculation of theminimum volume of mother liquor which must be purged from thecrystallizer that day to ensure that the product will meet purityspecifications during the next 24 hours. Larger purges result in acleaner product, but at the cost of reduced material efficiency and/orthe need to reprocess a greater amount of impure mother liquor material.Mother liquor purging also frequently results in large upsets in thecrystal size distribution, which may lead to rejection of some productfor reasons unrelated to its level of purity.

The demand for crystalline commodity chemicals is usually met withcontinuous crystallizers because of their low labor input andpredictable behavior, but the purity level of the product is geared bysimple economics toward the purity specifications of 90% of the market.Customers who may require more modest amounts of material with impurityconcentrations which are lower by several orders of magnitude areusually forced to repurify the commodity material themselves or obtainit from a specialty manufacturer of fine chemicals. Continuouscrystallizers are almost never used in these situations because of theirprohibitive cost and the intermittent nature of the market. For thesereasons the cost per pound of purified crystalline materials typicallyclimbs very steeply as purity specifications are tightened and potentialdemand for higher purity grades is curtailed accordingly.

It is an object of the invention to provide a countercurrent multistagecrystallization process for producing ultrapure crystalline compoundswherein dissolution and crystallization occur within each of a series ofmodular devices.

Another object of the invention is to provide inexpensive modulessuitable for performing the process of the invention and which can becombined in series to achieve any desired degree of purity with only amodest increase in the amount of labor required.

Other objects and advantages of the invention will be apparent from thefollowing description, drawings, and disclosure.

SUMMARY OF THE INVENTION

In accordance with the present invention, there is provided a processfor separating the desired crystalline compound from a mixture of solidsubstances which comprises disposing a solid mixture into the first of aseries of recrystallization units or apparatuses and solvent into thelast unit in the series, moving solid material progressively from thefirst unit to the last unit, and moving solvent progressively from thelast unit to the first unit to achieve multistage countercurrentrecrystallization. The desired crystalline substance achievesprogressively higher degrees of purity upon recrystallization at eachsuccessive stage, while the solvent has progressively higherconcentrations of impurities as it moves from the last to the firstrecrystallization unit. The number of stages required depends on thedesired purity and the properties of the crystalline material. At eachstage in the process, crystals from the prior stage are dissolved in aheated or higher temperature section of a unit, while higher puritycrystals are simultaneously recrystallized in another cooled or lowertemperature section of the unit. The temperature of the high temperatureregion in each unit is preferably substantially the same, and thetemperature of the low temperature region in each unit is preferablysubstantially the same. The process is accelerated by strategicpositioning of the high-temperature dissolution section relative to thelow-temperature recrystallization region to take advantage of naturalconvection. Because the process is dependent on differences insolubility as a function of temperature and does not rely on evaporationof solvent to achieve separation, the process is most efficient forpurification of crystallizable compounds whose solubility variessignificantly over the range of operating temperatures. Crystals havingthe desired purity are removed from the last unit on either anintermittent or continuous basis and solvent is removed from the firstunit either intermittently or continuously.

The crystalline material to be purified by the present invention caninclude any salt or other crystalline material which is less soluble asthe temperature of the solvent is decreased and whose saturated solutionin solvent is more dense than the pure solvent. Such crystallinematerials include inorganic salts such as CsI, CsCl, and NaI. Thepreferred solvent is water, although other solvents well known in theart can be used.

The present invention also provides an automated apparatus forcontinuous multistage recrystallization comprising individual modularunits. Each unit has a vessel for containing a solvent capable ofdissolving the desired crystalline substance, means for generating andmaintaining a temperature profile or gradient within the solvent toinduce recrystallization within a selected region of the vessel, andmeans for removing crystals from the vessel and conveying these crystalsto the next unit.

The present invention also provides an apparatus for batch-continuousmultistage recrystallization comprising individual modular units. Eachunit has a vessel for containing a solvent capable of dissolving thedesired crystalline substance, a first perforated container which can besuspended near the top of the vessel and is used for supporting crystalswhich are to be dissolved, a second perforated container positioned nearthe bottom of the vessel for collecting recrystallized material, andmeans for generating and maintaining a temperature profile or gradientwithin the solvent to induce dissolution of the crystals near the top ofthe vessel and to cause recrystallization near the bottom of the vessel.

The process of the invention has the advantage of providing a simple andefficient procedure for purifying a crystalline material to any desiredpurity level with a minimal amount of labor. The process also minimizesthe need for frequent sampling and analysis of various impurities. Theapparatus of the invention can generally be easily and inexpensivelyconstructed from commercially available materials. The apparatuses havethe additional advantage of being simple to operate, maintain andrepair, and accordingly are of great value in industrial practice.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic perspective of an automated recrystallizationapparatus in accordance with the invention;

FIG. 2 is a perspective view with portions cut away to reveal theinternal components of a manual recrystallization apparatus which can beused in accordance with the process of the invention;

FIG. 3 is another perspective view of the manual recrystallizationapparatus shown in FIG. 2 without portions cut away;

FIG. 4 is an exploded perspective view of the manual recrystallizationapparatus shown in FIG. 3;

FIG. 5 is a schematic for illustrating how the manual recrystallizationapparatus shown in FIG. 3 may be used in a multistage recrystallizationprocess; and

FIG. 6 is a graph showing the solubilities of various salts as afunction of temperature.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates an automated apparatus for purifying a crystallinematerial in accordance with the invention. The apparatus can include atleast three individual recrystallization units 10, 12, 14, 16, dependingon the properties of the material being purified and the desired degreeof purity. Each individual unit comprises a U-shaped tubular vessel 18capable of containing a non-hazardous solvent 19, such as water, inwhich the crystalline material is soluble. The units can be made ofglass, molded thermoplastic parts, and/or metals which are unaffected bycorrosion or chemical attack by ionic inorganic salts in aqueoussolution. Recrystallized material 20 forms along the inside walls of thevessel and accumulates at the bottom of the vessel. In a preferredembodiment of the invention, a heating well 22 is used to heat solventbefore it comes into contact with crystals that are to be dissolved inthe dissolving well 50. Each unit is provided with a jacket-type chilleror heat exchanger 24 for removing heat from the solvent at the cooled orlow temperature vertical leg 36 of the vessel. A chilled fluid such aswater enters an inlet 26 of the chiller, absorbs heat from the solvent,and exits from an outlet 28 of the chiller. Perforated hollow spheres 30are threaded on an endless loop of knotted rope 32 and drawn by a matingsprocket (not shown) down into a heated or high temperature vertical leg34 of the U-shaped vessel and up out of the cooled vertical leg 36.

Perforations 38 on the sphere scrape growing crystals from the innersurface of the tubular vessel and into the interior of the sphere.Crystals are conveyed up out of the solution while solution drains fromthe crystals through a small orifice or orifices (not shown) in thesphere wall, along the rope and back into the cooled vertical leg. Themaximum amount of solution is drained off the crystals.

A funnel-shaped receiver 46 is provided for collecting crystals whichare dumped from the hollow spheres at the apex of the loop of hollowspheres. Conveying means 48 are provided for transporting recrystallizedmaterial formed in one unit to the dissolving well of the next unit. Theconveying means could, for example, be chutes or mechanical conveyers.Each unit is provided with a filter 52 for removing accumulatedparticulate impurities such as polyanionic silicates, aluminates,borates, phosphates, arsenates, tungstates, and molybdates. Because manyof the aforementioned polyanionic impurities tend to polymerize slowlyin the neutral solution pH range where the units might be operated, itmay be wise to withhold filtration from the first unit in which they arefound in order to maximize the opportunity for the insoluble particlesto grow to a size large enough to be filtered off easily.

For purposes of this description, solution, solvent and mother liquormay, depending on the circumstances, be used interchangeably. Solventrefers to a liquid capable of dissolving the crystalline material,mother liquor refers to the solution from which material recrystallizes,and either solvent or mother liquor may be referred to as solution.

Solution is circulated through each unit, in a manner to be describedhereafter, by means of a recycle overflow 56, a solution manifold 62, aconduit 54, and a heated solution overflow 60. Solution is moved fromone unit to a preceding unit of the process or out of the process fromthe first unit by means of a mother liquor overflow 58.

The solubility of most salts, such as CsI, KI, and NaI, increases athigher temperatures. Accordingly, most salts will preferentiallycrystallize in the cooled vertical leg of each unit, which will causethe salt concentration of solution contained within each cooled verticalleg to be lower than in the corresponding heated vertical leg. Thisconcentration difference causes solution within the heated vertical legto have a higher density than the solution within the cooled verticalleg, which results in the liquid level of the cooled vertical leg beinghigher than that of the heated vertical leg. For purposes ofillustration, the differences in liquid level have been exaggerated inFIG. 1.

By reversing the temperature gradient in each unit, crystalline materialwhich is more soluble as the temperature of the solvent is decreased canbe purified.

Hot and cold operating temperatures of 40° C. and 20° C., respectively,can generally be used. If the automated apparatus is made of glass,metal, or appropriate plastic, it should be possible to operate at a hottemperature up to the boiling point of the solution and at a coldtemperature down to the freezing point of the solution. Operation of therecrystallizing units at elevated temperatures and/or pressures may bedesirable in selected cases.

A crystallizable material that is to be purified in accordance with theinvention using the apparatus of FIG. 1 is introduced as a solid intothe dissolving well 50 of the first unit 10, where the material isdissolved without the use of mechanical agitation. Raw material to beprocessed may be introduced either periodically or continuously. A smallquantity of mother liquor periodically or continuously overflows fromthe cooled vertical leg 36 of each of the intermediate units 12, 14 viathe mother liquor overflow 58 and enters the heated vertical leg 34 ofthe preceding unit at the heated solution overflow 60. At the last unit16 a small quantity of high purity solvent enters the heated verticalleg through the fresh solvent inlet 94, while a similar quantity ofmother liquor containing a high concentration of impurities exits thefirst unit 10 at the mother liquor outlet 96. The majority of motherliquor overflowing from the cooled vertical leg of each unit enters thefilter 52 via the recycle overflow 56, passes through the filter toremove sparingly soluble particulate impurities, and enters the solutionmanifold 62. Manifolds 56 and 58 are positioned and/or valved toaccommodate overflow and recycle flow rates which are appropriate for agiven process.

Liquid flow through the manifolds, heating wells 22, and dissolvingwells 50 is controlled by free convection. Heated solution passing downthrough the dissolving well of each unit dissolves crystals containedtherein, and becomes saturated with the crystallizable material. Thesaturated solution has a higher density than the unsaturated motherliquor overflowing from the cooled vertical leg, and consequently flowsdownward through the dissolving well. The relatively cool unsaturatedmother liquor overflowing from the cooled vertical leg and the warmersaturated solution from the dissolving well became intermixed at a firstfluid junction 98. At a second fluid junction 99 the flow splits with aportion passing up the heating well 22, across a conduit 54 and backinto the dissolving well to complete a convection driven looped flow,while another portion of the solution at fluid junction 99 rises up themanifold and passes into the heated vertical leg 34 via the heatedsolution overflow. The solution that overflows into the heated verticalleg is nearly saturated, whereas the solution at some intermediate pointbetween the cooled vertical leg and the heated vertical leg becomessupersaturated, causing recrystallization. The colder solution n thecooled vertical leg, having been depleted of crystallizable material,has a lower density than the solution in the heated vertical leg, whichcauses solution to overflow from the cooled vertical leg into themanifold and ultimately back into the heated vertical leg.

Crystals formed in the last vessel 16 are of a higher purity than theraw crystals deposited in the dissolver 50 of the first unit 10.Crystals formed at each unit are scraped out of the vessel by theperforated hollow spheres 30, dumped into the funnel-shaped receiver,and conveyed to the dissolving well of the next unit. At each unit thecrystals formed in the U-shaped tubular vessel are of a higher puritythan the crystals which entered the dissolver. A high purity crystallineproduct exits the apparatus via the product chute 97.

Referring to FIG. 3, in accordance with another aspect of the inventioneach manually operated recrystallization unit in a batch-continuousmultistage system has a recrystallization section 64 and a filtrationsection 92.

Referring to FIGS. 2 and 4, crystals are suspended in a perforatedcontainer 68, having a perforated lid 72 near the top of a vessel 66capable of containing a solvent into which the crystals can bedissolved. An outer retaining vessel 75 is preferably provided tocollect spillage and/or leakage from the apparatus. A second perforatedcontainer 70, having a perforated lid 74, is positioned near the bottomof the vessel. These various modules are made of thermoplastic moldedparts (preferably, high molecular weight polyethylene or polypropylene)which are inert with respect to corrosion or chemical attack by ionicinorganic salts in aqueous solution. A heater 78 having a heatingelement 81 which extends a significant fraction of the full height ofthe vessel along a line parallel to the axis of the vessel is submergedwithin a cylindrical dip tube 76 which also extends along the fullheight of the vessel and is positioned near the inner wall of thevessel. Triangular notches 79 (only one is shown) are provided at thebottom of the dip tube to allow fluid communication between the vesseland the dip tube. Solution is drawn from the dip tube through a tube 84to a pump 80, and is pumped through tubing 87 to a first filter 82containing a carbon canister and through a second filter 83 containing awound filter cartridge, and then into the upper perforated container 68via tubing 86. Each perforated container 68, 70 is fitted with aperforated downflow pipe 89, 91 which extends vertically along thecentral axis of each of the containers. When both containers areproperly positioned in the vessel and coupled, the two downflow pipesact as a single pipe having perforations within each of the containers.

Tubing 88, 90 is wound around the downflow pipe of each vessel, witheach end of said tubing extending out of the vessel and provided withmeans for circulating a heating or cooling fluid through the tubing. Thetubing is preferably made of plastic.

The containers 68, 70, vessel 66, and heating, cooling, and filtrationaccessories are constructed to facilitate easy assembly and disassemblyof the component parts to permit removal of the containers asillustrated in FIG. 4.

The manual recrystallizer is operated by submerging an empty perforatedcontainer 70 with a perforated lid 74 into the bottom of the vessel 66.The crystalline material which is to be purified is deposited intoanother perforated container 68 and a perforated lid 72 is fitted ontothe container 68. The perforated container 68 is then lowered into thevessel 66 and positioned above the empty container 70. The solventsolution level should be high enough to completely immerse the crystalsin the container 68. A heated fluid is pumped through the heating coils95 of the upper container holding the crystals to cause the temperatureof the solution in the upper portion of the vessel 66 to rise relativeto the temperature in the lower portion of the vessel. This temperaturedifference is further enhanced by circulating a chilled fluid throughthe coiled tubing 90, 93 of the lower perforated container. The heatedsolution in the upper portion of the vessel causes the crystals in theupper perforated container to dissolve without the aid of mechanicalagitation. As the crystals dissolve, the heated solution becomes moredense than the solution in the lower portion of the vessel and sinks tothe bottom of the vessel via the perforations in downflow pipe 89 andperforations in the bottom of the upper container and in the lid of thelower container. Hot and cold operating temperatures of 40° C. and 20°C., respectively, have been found satisfactory for recrystallizing CsIfrom aqueous solutions with plastic equipment of the type described.

Recrystallization occurs in the lower container near the cooling coils93. During the process, sparingly soluble particulate impurities, suchas Fe, Si, and Al, are removed from the solution by filtration. Solutionenters the dip tube 76 through triangular notches 79 (only one of thenotches is shown) and is pumped up through the dip tube, which has aheating element 81 running through it. The heater heats the solution byperhaps 10°-20° C. and prevents recrystallization from occurring in thepump 80 and the filters 82, 83. After being filtered, the heatedsolution is returned to the top of the vessel. During the process, freeconvection is taken advantage of to accelerate dissolution andrecrystallization. Denser solution having a high concentration of thedissolved crystalline material flows downward along the axis of thevessel while the cooled solution near the bottom rises along the wallsof the vessel.

A series of manual recrystallizers may be operated in batch-continuousmanner, with crystals and solvent moving countercurrently from vessel tovessel to achieve any desired degree of purity. A multistage processusing five manual recrystallizers is illustrated in FIG. 5.

At each stage crystals, are dissolved in the upper containers 68 andrecrystallized in the lower containers 70. At the conclusion ofrecrystallization, the containers are removed and solution is allowed todrain back into the vessel from which it came. A predetermined quantityof solution (perhaps 200-1000 ml) from the vessel of the firstrecrystallization unit 114 is removed from the process, as indicated bythe path 132, to control the accumulation of impurities. A similarrelatively small quantity of solution from each of the otherrecrystallization units 116, 118, 119, and 120 is transferred, asindicated by the paths 128, to each of the preceding units, with atleast a portion (and generally all) of this solution being used to rinsethe crystals which were removed from the recrystallization unit intowhich the solution is being transferred. By recycling mother liquor ascrystal wash in a flow countercurrent to the stream of crystalline solidmaterial, efficiencies approaching 100% can be achieved by allowingimpurities to accumulate to very high levels in the mother liquor of thefirst crystallizer before recycling or discarding it. Fresh solvent isadded to the last recrystallization unit 120, as indicated by the path130, to replace solution transferred to the fourth unit 119. The timeallowed for drainage of mother liquor from wet crystals, the volume ofcleaner mother liquor used for washing, and the technique used forpouring it through the crystals all have an influence on thedistribution of a given impurity in the mother liquor throughout theseries of units. If a larger volume of wash water is needed to maintainan acceptable distribution of impurities, a larger volume of deionizeddouble-distilled water must be added to the last unit and a comparableamount of mother liquor removed from the first unit.

Since the upper containers 68 and the lower container 70 aresubstantially identical and functionally interchangeable, their rolescan be reversed. This eliminates any need to transfer recrystallizedmaterial from one container to another. Consequently, for the nextdissolution-recrystallization cycle, the empty containers 68 arepositioned at the bottom of each vessel. The bottom container holdingthe crystals formed in the first unit 114 is placed in the uppercontainer position of the second unit 116. This transfer is indicated bythe path 126. Likewise, the crystals from the other units arerespectively placed in the upper container positions of the next unit.These transfers are indicated by the paths 126. The container holdingthe purified crystalline product formed in the last unit is drained andthe contents thereof are transferred via path 138 to drum 122. Eachcontainer is marked to ensure that its use is restricted only to twoadjacent vessels in order to minimize the opportunity for undissolvedcrystals or residual solution to contaminate the much cleaner materialin units closer to the final purified product. The above steps arecontinually repeated to provide the desired countercurrentbatch-continuous recrystallization process.

EXAMPLE

The purification of cesium iodide was carried out using thecountercurrent batch-continuous manual process and apparatus describedhereinabove. Deionized double-distilled water was used as the solvent.The cesium iodide to be purified originally contained approximately 30to 1000 ppm of sodium. Product CsI with sodium at far less than 0.5 ppmand overall CsI purity between 99.999% and 99.9999% was achievedconsistently when Na concentrations in units 5, 4, 3, 2, and 1 were notpermitted to exceed 3, 20, 130, 800, and 5,000 ppm, respectively. If anyof these concentrations were reached during periodic sampling andanalyses, mother liquor from the first (and dirtiest) unit was removedfor reclamation (typically amounting to 5-10% of cumulative feed,depending on its purity); the other four mother liquors were dilutedsomewhat and filtered into the previous unit after it was emptied, andthe last unit was filled with deionized, double-distilled water beforeoperation was resumed.

Again referring to FIG. 5, polished cakes of cesium iodide, having apurity of 99.9%, were removed from a plastic-lined storage drum 112,broken into chunks, and transferred to the upper container of the firstrecrystallization unit 114 as indicated by the path 136. Alternatively,cesium iodide in any form suitable for depositing into the containers 68and having a purity of 99.9% can be introduced into the process asindicated by the alternate path 134.

CsI is particularly suitable as the crystallizable material to bepurified by the method of the present invention. One reason for this isthat CsI has a relatively steep solubility curve, as shown in FIG. 6.That is, the solubility of CsI in water increases rapidly in thetemperature range of interest (10°-70° C.). It is believed that themethod of the present invention will work particularly well for any saltwith a solubility curve as steep as, or steeper than, that of CsI. Ifthe solubility curve is steep, less heat and less change in solutiontemperature are needed to achieve an equivalent amount ofrecrystallization. The method of the present invention cannot beutilized, as a practical matter, with a crystallizable material, thesolubility of which does not change appreciably with change in solventtemperature within the temperature range of interest. Such a material isNaCl, which, as shown in FIG. 6, does not appreciably change solubilitybetween 0° C. and 100° C.

While the invention has been described in connection with what arepresently considered to be the most practical and preferred embodiments,it is to be understood that the invention is not to be limited to thedisclosed embodiments but, on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims, which scope is to be accorded the broadestinterpretation so as to encompass all such modifications and equivalentstructures.

What is claimed is:
 1. A multistage recrystallization process forpurifying a crystalline material, said process comprising the stepsof:(a) providing a series of vessels, said series including at least afirst vessel and a last vessel, each vessel containing solvent andhaving a first temperature region and a second, different temperatureregion; (b) dissolving crystalline material in the first temperatureregion of said first vessel; (c) crystallizing dissolved material in thesecond temperature region of said first vessel; (d) removingrecrystallized material in solid form from the second temperature regionof said first vessel; (e) transporting said removed recrystallizedmaterial to the first temperature region of the second vessel anddissolving said removed recrystallized material therein; (f)crystallizing dissolved material in the second temperature region ofsaid second vessel; (g) transferring a portion of the solvent from whichmaterial has recrystallized in said second vessel to said first vesseland retaining a portion in said second vessel; (h) repeating steps (b),(c), (d), (e), (f), and (g) through the said series of vessels,including introducing crystalline material into said first vessel,moving said crystalline material in recrystallized form progressively tosaid last vessel, introducing solvent into said last vessel, movingsolvent containing impurities progressively to said first vessel, andremoving solvent containing impurities from said first vessel, thesolvent being introduced into said last vessel containing fewerimpurities than the solvent being removed from said first vessel; and(i) separating solid impurities from the solvent which is in said seriesof vessels.
 2. The process of claim 1, wherein the first temperatureregion is at a higher temperature than the second temperature region. 3.The process of claim 1, wherein the first temperature region is at alower temperature than the second temperature region.
 4. The process ofclaim 2, wherein temperature control means are utilized in each of saidvessels to maintain the first temperature region at a higher temperaturethan the second temperature region.
 5. The process of claim 2, whereinheating means and cooling means are utilized in each of said vessels tomaintain the first temperature region at a higher temperature than thesecond temperature region.
 6. The process of claim 5, wherein thecrystalline material has a solubility curve at least, as steep as thatof cesium chloride.
 7. The process of claim 5, wherein the crystallinematerial has a solubility curve at least as steep as that of cesiumiodide.
 8. The process of claim 2, wherein the crystalline material isselected from the group consisting of CsI, CsCl, and NaI.
 9. The processof claim 5, wherein the crystalline material is selected from the groupconsisting of CsI, CsCl, and NaI.
 10. The process of claim 1, wherein insteps (d), (e), (f) and (g), solvent being transferred from said secondvessel to said first vessel is utilized to wash the recrystallizedmaterial being removed from said first vessel, the crystal wash goinginto said first vessel.
 11. The process of claim 1, wherein in step (g)the portion of the solvent being transferred is only a minor portion.12. The process of claim 1, wherein said series of vessels consists offive or fewer vessels.
 13. The process of claim 1, wherein therecrystallized material being produced by the process has a plurality inexcess of 99.9 percent.
 14. The process of claim 13, wherein therecrystallized material is an inorganic salt.
 15. The process of claim1, wherein the crystalline material is selected from the groupconsisting of CsI, CsCl, and NaI, and the recrystallized material beingproduced by the process has a purity in excess of 99.999 percent. 16.The process of claim 15, wherein the crystalline material is selectedfrom the group consisting of CsI and CsCl, and the concentration ofsodium in the first, second, third, fourth, and fifth vessels does notexceed 5000, 800, 130, 20, and 3 pm, respectively.
 17. The process ofclaim 1, wherein in step (g) the amount of solvent being transferred isthe minimum amount necessary to maintain steady state operation.
 18. Theprocess of claim 1 which consists essentially of steps (a), (b), (c),(d), (e), (f), (g), (h), and (i).
 19. A multistage recrystallizationprocess for purifying a crystalline material, said process comprisingthe steps of:(a) providing a series of vessels, said series including atleast a first vessel and a last vessel, each vessel containing solventand having a first temperature region and a second, differenttemperature region; (b) dissolving crystalline material in the firsttemperature region of said first vessel; (c) crystallizing dissolvedmaterial in the second temperature region of said first vessel; (d)removing recrystallized material in solid form by automated means fromthe second temperature region of said first vessel; (e) transportingsaid removed recrystallized material by automated means to the firsttemperature region of the second vessel and dissolving said removedrecrystallized material therein; (f) crystallizing dissolved material inthe second temperature region of said second vessel; (g) transferring aportion of the solvent from which material has recrystallized in saidsecond vessel to said first vessel and retaining a portion in saidsecond vessel; (h) repeating steps (b), (c), (d), (e), (f), and (g)through the said series of vessels, including introducing crystallinematerial into said first vessel, moving said crystalline material inrecrystallized form progressively to said last vessel, introducingsolvent into said last vessel, moving solvent containing impuritiesprogressively to said first vessel, and removing solvent containingimpurities from said first vessel, the solvent being introduced intosaid last vessel containing fewer impurities than the solvent beingremoved from said first vessel; and (i) separating solid impurities fromthe solvent which is in said series of vessels.
 20. The process of claim19, wherein in step (g) the portion of the solvent being transferred isonly a minor portion.
 21. The process of claim 19, wherein therecrystallized material being produced by the process has a purity inexcess of 99.9 percent.
 22. The process of claim 19, wherein therecrystallized material being produced by the process has a purity inexcess of 99.999 percent.
 23. The process of claim 19, wherein in step(g) the amount of solvent being transferred is the minimum amountnecessary to maintain steady state operation.
 24. The process of claim19, which consists essentially of steps (a), (b), (c), (d), (e), (f),(g), (h), and (i).